1. Field of the Invention
The invention mainly relates to a lithium silicate-based compound that is usefully employed as a positive electrode active material of lithium ion batteries. The invention further relates to a lithium ion secondary battery in which the lithium silicate-based compound is used as an active material.
2. Description of the Related Art
The lithium ion secondary batteries are small-sized batteries that are superior in energy density and extensively used as power supplies of portable electronic devices. A positive electrode active material typically used in the lithium ion secondary batteries is layered compounds including, for example, LiCoO2. In these compounds, however, oxygen is easily desorbed at the temperatures around 150° C. when the batteries are fully charged. This is likely to cause the oxidation exothermic reaction of non-aqueous electrolytic solutions, possibly endangering the safety of batteries.
The positive electrode active materials introduced in recent years are phospho-olivine compounds, LiMPO4 (for example, LiMnPO4, LiFePO4, LiCoPO4). These compounds are attracting attention because of the advantages; improvement of thermal stability through divalent/trivalent oxidation-reduction reaction instead of trivalent/tetravalent oxidation-reduction reaction in which an oxide, such as LiCoO2 is used as a positive electrode active material, and high discharge voltages obtained by arranging the polyanions of hetero elements having high electronegativity around the central metal.
However, the positive electrode materials including the phospho-olivine compounds have some disadvantages. Due to an average discharge voltage as low as 3.5 V or lower and a heavy formula weight of phosphorous polyanions, the theoretical capacities of these positive electrode materials are limited to about 170 mAh/g. With the lithium ion secondary batteries in which LiCoPO4 or LiNiPO4 is used and operation voltages are higher than 4.5 V, it is not possible to find electrolytic solutions capable of withstanding such high charging voltages. With the lithium ion secondary batteries in which LiMnPO4 is used, there is an unsolved problem; severe degradation of cycle characteristics. With the LiFePO4 batteries that are more advantageous than the other batteries in practical use, there is an unsolved problem too; capacity reductions caused by the oxidation of divalent iron.
JP 2001-266882 A discloses lithium silicate-based compounds containing Li2FeSiO4 (theoretical capacity: 331.3 mAh/g) and Li2MnSiO4 (theoretical capacity: 333.2 mAh/g) as examples of the positive electrode active material. These positive electrode active materials are the inexpensive and environmental load-reducing materials that are abundantly available. Other advantages of the positive electrode active materials are; large theoretical lithium ion charge/discharge capacities, and no release of oxygen at high temperatures. The lithium silicate-based compounds, which consist of only the metal elements that are abundantly available and obtainable at low prices, are the environmental load-reducing materials with large theoretical lithium ion charge/discharge capacities, wherein oxygen is not released at high temperatures. Thus, the lithium silicate-based compounds having these advantages are prospective positive electrode materials for the next-generation lithium ion secondary batteries.
However, there is still a concern for stability even with Li2FeSiO4 in which iron (Fe) is divalent because oxidation possibly causes the reductions of battery capacities. It is disclosed in JP 2011-014445 A that LiFeSi2O6 is used as a positive electrode active material. The compound disclosed in this document, in which iron (Fe) is trivalent, is expected to have a good stability. On the other hand, it is disclosed in this document that the first discharge capacity of the lithium ion secondary battery containing LiFeSi2O6 as its positive electrode active material was 102.9 mAh/g. This discharge capacity is not as large as the result of the comparative example in which LiFePO4 is used.